Power cords and extension cords are two different things. A power cord is a cable that connects your electrical device to an outlet, while an extension cord is a cable that connects two or more outlets. They have different purposes and have different features.
A power cord has a plug on one end and a receptacle on the other. An extension cord has one end that has a plug, and the other end has multiple plugs so you can connect it to more than one outlet.

An extension cord can be used for a variety of tasks, such as connecting your computer to an outlet so you can save energy, connecting large appliances to the wall to avoid tripping over them, or powering multiple devices from one outlet.